In recent years, research and development have been actively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a layer including a substance having a light-emitting property is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the substance having a light-emitting property.
Since such light-emitting elements is of self-light-emitting type, it is considered that the light-emitting elements have advantages over liquid crystal display devices in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, other advantages of such light-emitting elements are that the elements can be manufactured to be thin and lightweight and the response speed is very high.
Since such light-emitting elements can be formed into a film shape, plane light emission can be easily obtained by forming a large-area element. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a planar light source applicable to illumination and the like.
Light-emitting elements using electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a substance having a light-emitting property.
When an organic compound is used as a light-emitting substance, electrons and holes are injected into a layer including a light-emitting organic compound from a pair of electrodes by voltage application to a light-emitting element, so that current flows therethrough. The electrons and holes (i.e., carriers) are recombined, and thus the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on this mechanism, such a light-emitting element is called current excitation type light-emitting element.
It is to be noted that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a material used, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.
For example, in Reference 1, a hole-blocking layer is formed in a light-emitting element, so that the light-emitting element using a phosphorescent material can emit light efficiently. However, such a hole blocking layer has poor durability as described in Reference 1 and the light-emitting element has short lifetime (Reference 1: Tetsuo Tsutsui and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-L1504 (1999)). Thus, a light-emitting element having longer lifetime is desired.